Characteristics of the organism and its antigens

Figure 1 is a representation of the schistosome life cycle. A phase of sexual maturation and mating in the definitive mammalian host alternates with a phase of asexual replication in an intermediate mol-luscan host, i.e. freshwater snails of the genera Biom-phalaria, Bulinus and Oncomelania for S. mansoni, S. haematobium and S. japonicum, respectively.

The free-living cercaria is released from infected snails and swims actively in fresh water for several hours. Upon contact with the skin of an appropriate mammalian host it penetrates directly through the skin, sheds its tail and outer glycocalyx coat and transforms into the young schistosome or schistoso-mulum. After a 2 day sojourn in the skin of experimentally infected animals the schistosomulum enters the bloodstream. The larvae can be found in large numbers in the lungs of experimental hosts between 4 and 7 days after skin penetration, but it is believed that during this time they are circulating throughout the body, lodging only temporarily in highly vascul-ated tissues such as the lungs. From 7 to 10 days after infection the larvae are found in increasing numbers in the hepatic portal system. Maturation and pairing of the sexually differentiated worms occurs, and the paired worms take up their final position in the small mesenteric veins (S. mansoni and S. japonicum) or the veins of the vesical plexus (S. haematobium).

The mean lifespan of the adult worm is estimated to be 2-8 years, depending on species, but there is evidence of S. mansoni worms being able to survive 2-3 decades. Egg-laying starts about 6 weeks after infection, the females releasing between 300 eggs {S. mansoni) and 3000 eggs (S. japonicum) a day. A proportion of the eggs released by S. mansoni and S.

Veins of bladder ■ S hFtamfllpbkjm

Mesenteric vessels ■ S_ mansan ol bowsl ■ S, japonicum

Veins of bladder ■ S hFtamfllpbkjm

Mesenteric vessels ■ S_ mansan ol bowsl ■ S, japonicum

Hcpatic portal vessels

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